Pipelines are constructed and laid using various types of laying vessels, depending on the depth of the bed. One type of vessel, suitable for relatively shallow beds, is configured to join pipe sections on a substantially horizontal assembly line, and is equipped with a so-called S-lay ramp—named after the shape the pipeline assumes between the laying vessel and the bed. Another type of vessel, suitable for deep beds, is configured to join pipe sections on a substantially vertical assembly line, and is equipped with a so-called J-lay ramp—also named after the shape the pipeline assumes between the laying vessel and the bed.
In both cases, the portion of the pipeline between the laying vessel and the bed is subjected to bending stress (i.e., forms one curve using the J-lay system, and two curves using the S-lay system). Until it is actually put into operation, the pipeline is obviously empty, so the external pressure, particularly in deep water, is much higher than inside the pipeline. Pipelines may be laid at depths of as much as 3000 meters (9842.52 feet) below sea level, so sharp curves combined with the difference between the external and internal pressure may crush and rupture the pipeline, thus resulting in flooding, which has serious consequences. In particular, at least part of the pipeline is irreparably damaged and must be replaced; and clearing the flooded pipeline is an expensive, extremely time-consuming job.
One known solution to the problem is to insert and move a plug along the pipeline, to prevent the portion of the pipeline upstream from the plug from flooding as the pipeline is eased onto the bed.
The plug is moved along the pipeline using various techniques. For example, in documents U.S. Pat. No. 4,498,811 and PCT Patent Application No. WO 03/042594, it is drawn along using a cable inserted inside the pipeline and operated by a winch on the laying vessel. In another technique known as pigging, the plug is blown along by compressed air generated at a pumping station, for example located at one end of the pipeline. In document PCT Patent Application No. WO 2007/040403, the plug is moved along the pipeline by the difference in pressure up and downstream from the plug. In another technique, the plug is associated with a self-propelled carriage, but this requires an independent power source, as described in documents U.S. Pat. No. 3,837,214 and U.S. Pat. No. 3,978,678.
Each of the above plugging techniques has pros and cons in terms of plug positioning precision, operating range, cost, production and installation.
Moreover, the methods in the above documents are ineffective in preventing crushing of the pipeline from spreading and so endangering the integrity of a large section of the pipeline and the plug, and the sealing capacity of the plug.